Attempt to make plone.api importable to Sphinx by using its system Python instead of the Python in the Makefile

Author: stevepiercy

Makefile

@@ -234,7 +234,7 @@ rtd-pr-preview:  ## Build pull request preview on Read the Docs
 	ln -s ../submodules/volto/docs/source ./docs/volto
 	ln -s ../submodules/plone.restapi ./docs/plone.restapi
 	ln -s ../submodules/plone.api/docs ./docs/plone.api
-	cd $(DOCS_DIR) && $(SPHINXBUILD) -b html $(ALLSPHINXOPTS) ${READTHEDOCS_OUTPUT}/html/
+	cd $(DOCS_DIR) && sphinx-build -b html $(ALLSPHINXOPTS) ${READTHEDOCS_OUTPUT}/html/
 
 .PHONY: storybook
 storybook: